Years ahead of the Soviet Union’s last-minute entry into war against Japan towards the end of World War II, the country massively fortified its position along the border area between Mongolia and Manchuria.

The finding is based on expeditions by a joint team of Japanese and Mongolian researchers to the former battle site of what is known as the 1939 Nomonhan Incident in Japan. Russia today calls it the Battle of Khalkhin Gol.

“Our study showed that the Soviet Union was covertly preparing for a massive assault on Japan even years before it entered war against Japan (in August 1945),” said Hisaya Okazaki, who led the research team.

The recent expedition in Mongolia was carried out in June to mark the 75th anniversary of the Nomonhan Incident.

Mongolia was under the Soviet sphere of influence at that time. As a puppet state of Japan, Manchukuo in Manchuria, in northeastern China, was controlled by the Kwantung Army, a division of the Imperial Japanese Army.

A skirmish between Mongolian and Manchurian troops that broke out on May 11, 1939, escalated into a full-scale battle between Soviet troops and the Kwantung Army. After the Soviet forces’ massive assault on Aug. 20, the Japanese army was wiped out. By the time a cease-fire was reached on Sept. 16, Japanese casualties totaled 20,000 and 26,000 for the Soviet side.

In the recent expedition, the researchers confirmed two former positions built by the Soviet forces--Matad and Sanbeis, which is known as Choibalsan today.

Tamsagbulag, another Soviet position, was confirmed five years ago on the researcher’s previous trip. While the existence of these former Soviet bases had been speculated, their exact locations and details of the facilities had yet to be confirmed until the latest discovery.

Tamsagbulag and Matad are 13 kilometers east to west and 10 km north to south. Sunbeis is 26 km by 29 km. The size of each is larger than the space inside Tokyo’s Yamanote Line, which encircles the city’s central area.

The geography of the locations, which are tucked away in rolling hills, attests to the fact that the Soviet forces picked the sites to elude their adversary.

Remains of rail tracks, barracks for high-ranking officers, batteries, pillboxes, firing ranges, cemeteries with nameless graves, warehouses made of tents and trenches that were used to relay instructions among troops were documented by the researchers.

Among the articles they found on the latest expedition were Soviet soldiers’ iron helmets first used in 1940, fragments of the 1943 jeeps that the United States provided to the former Soviet Union, and cartridge cases with engraved marks showing they were made in the early 1940s.

According to archives in Japan, Japanese aircraft did not observe any military facilities, such as a control tower, a hangar or a building, in Tamsagbulag during a flight over the area in June 1939.

As a result, the research team concluded that the positions were constructed in the 1940s.

Accounts by a local support the team's conclusion.

“Soviet troops built the positions while my grandfather was still alive,” said Gombosuren Bold, 39, a sole nomad living in Matad. “I was told that it was the site for fielding troops and soldiers, from where the Soviet forces made a sortie on the Japanese troops.”

The researchers also believe that the main force of the Soviet Zabaikal theater army was stationed in the area when the Soviet Union declared war against Japan on Aug. 8, 1945.

While the Nomonhan Incident is relatively unknown, it had significant ramifications on Japan’s future.

After the Kwantung Army’s defeat, factions pushing for Japan to advance southward were able to gain momentum. This culminated in the nation entering the Pacific War against the United States in 1941.

Although the Japanese officers defied Tokyo headquarters’ orders by escalating the Nomonhan Incident, no one was ever held accountable.

Those officers went on to play an important role in charting out disastrous campaigns, including the Battle of Imphal in India and Guadalcanal Campaign in Solomon Islands in the South Pacific. With little or no supplies, most of their troops died of starvation or disease in both battles.